Field of the Invention
This invention relates to a digital signal reproducing apparatus which is applied to a digital VTR.
Description of the Prior Art
There has been known a digital VTR which is configured to convert a composite color video signal into a digital signal and to record or reproduce it by means of a rotary head. In order to process a reproduced signal, a clock signal synchronized with the reproduced signal is reproduced by a PLL (phase locked loop). In FIG. 1 showing a conventional arrangement, reference numeral 41 refers to an input terminal of the reproduced signal, numeral 42 denotes the PLL within the broken line, and numeral 43 designates a decoding circuit of an M square code. The PLL 42 is composed of a phase comparator circuit 44 and a VCO (voltage controlled oscillator) 45. The phase comparator circuit 44 compares the reproduced signal with an output signal of the VCO 45 and supplies its comparison output to the VCO 45 as a control signal. The PLL 42 limits the lock range to a narrow range in order to prevent a pseudo lock. An output signal (clock signal) of the PLL 42 and the reproduced signal are fed to the decoding circuit 43, and decoding of the M square code is performed. The M square code is advantageous in that convergence of the spectrum of coded data is good and further in that no DC component is included as shown in FIG. 2. The PLL 42 is designed to have a narrow lock range 46 about the center frequency FO of the spectrum. Decoding of the M square code requires a clock signal having a frequency twice the data rate.
The digital VTR is operative not only in a normal playback mode, in which the tape speed is the same as that at the time of recording, but also in a variable speed playback mode, in which the tape speed is .+-. several times to .+-. decade times the speed in the normal playback mode (.+-. means the tape running direction). Regarding the variable speed playback mode, an operation in which the tape speed is set in a range of (-1 to +3) times is referred to as "variable", an operation in which the tape speed corresponds to the rotation speed of a jog dial is referred to as "jog", and an operation in which the tape speed is continuously variable in a range faster than the variable operation is referred to as "shuttle". The variable speed playback is used for the purpose of locating in a short time a desired one of the video signals recorded on a tape.
In order to prevent large changes in the data rate (frequency) of the reproduced signal during the variable speed playback, the conventional apparatus varies the rotation speed of a head drum depending on the tape speed X. As shown in FIG. 3A, the rotation speed R of the drum is controlled, so that changes in the frequency Fp (FIG. 3B) of the reproduced signal are limited to small values even when the tape speed of the variable speed playback is the maximum value .+-.40. In FIG. 3A, the range, in which the rotation speed R of the drum is constant, corresponds to the variable range of the tape speed X during the variable operation and the jog operation.
The method of varying the rotation speed of the drum to prevent large changes in frequency of the reproduced signal has a drawback in that the drum rotating mechanism makes it difficult to extend the range of the variable speed playback. For example, in the case of variable speed playback at a tape speed of .+-.100 times normal speed, it is difficult to prevent large changes in frequency of the reproduced signal by mere control of the rotation speed of the drum.